Local stresses at the interfaces are analyzed to make clear the role of the liquid phase in superplasticity for metal matrix composites. The local stress caused by sliding at the interfaces is higher than the applied local stress at the interfaces for the composites whose maximum elongation is attained in a state containing a liquid phase, however, the relationship is the opposite for the composites whose maximum elongation is attained in a solid state containing no liquid phase. Therefore, it is suggested that when the stress concentrations are caused, a liquid phase is needed in order to relax the stress concentrations, however, when no stress concentrations are caused, a liquid phase is not needed for superplasticity. This view was supported by investigation of cavitation.